Ride height will vary greatly from car to car as parts wear differently. On my cars, with rebuilt suspension and lowering springs, I like to run around 13.75-14.0 inches from center of the wheel to the bottom of the fender lip. Tire size is irrelevant unless you're measuring off the pavement, which is a no-no. Stock height is more like 15.0 front, 15.5 rear. Also, the rear will change about 0.5 inch between full tank of fuel and empty tank. I set my cars up so it's level (13.75 front and rear) with a full tank of fuel, and the back end lifts to about 14.25 when it's near empty. That allows for some leeway to carry passengers, luggage, etc...
Click the "www" button at the bottom of my post for photos of my two 1987 W124's in various states of lowering, etc.
